Keying circuit



April 1, 1941. HAHNDdRF 2,237,101

' KEYINe,cIRcuiT Filed Dec. 8, 1959 70 {0A0 COMPENSATOR (FOR smw-BrPER/00.)

AAAAAK INVENTORAI FRITZ HAHNDORF .ATTORNEY Patented Apr. 1, 1941 thatstares earner easier.

KEYING cmcm'r of Germany Applicatien December 8, 1939, Serial No.308,229

In Germany November 15, 1938 2 Claims.

The present invention relates to a circuit arrangement in which twoinductive resistances connected in parallel arrangement to the samedirect voltage source are to be disconnected simultaneously. Such acircuit arrangement exists for instance in the case of relays placed inparallel and connected to a direct voltage source across a switch.

In such an arrangement it was found that after disconnecting the directvoltage source the relay contacts retain for an appreciable length oftime the positions which they had at current passage so that asufliciently rapid switching is impossible.

In accordance with the present invention this drawback is avoided inthat the inductive resistance having a lower ratio between self induc-An example of construction according to the invention is shown in thefigure in connection with a key relay arrangement of a telegraphtransmitter. er potential and the key switch T have placed in seriesthereto the parallel connection of a relay coil A and a seriesconnection of a further relay coil B and rectifier G. The rectifierwhich may be a dry contact rectifier for instance is inserted with apolarity such that the anode of the rectifier is toward the positivepole of the direct voltage source Q. For the normal direction of thecurrent, therefore, the rectifier represents a negligibly lowresistance.

The switch contact a actuated by the relay A serves for introducingduring the switching pauses an additional load resistance. Hence, it

produces a load compensation and prevents an excessive increase of thevoltage. The switch contact b actuated by the relay B serves however forthe actual keying of the transmitter in that for instance the gridbiasing potential of a tube of the transmitter is shifted to such a highof thick wire wound on an iron core of relativeiy large cross sectionand adapted to operate pow erful contact members. The self induction ofthe coil therefore is very high while the D. C. resistance is low. Thecoil of the relay B requires a smaller number of turns of thinner wirewound The direct voltage source Q of lowon an iron core of relativelysmall cross section, so that the self induction is substantially lowerbut the ohmic resistance higher than in the case of the coil A. Thecontacts of this relay are so designed that only a low power will beswitched but with very low inertia. The relay A may be designed as aslow-acting-slow release relay and the relay B as a rapid relay.

In the absence of the rectifier G, a current induced by de-energizationof the core of relay A would flow through the coils A and B at themoment in which the key T is opened. This deenergization current throughthe coil A has the same direction as the normal current, but in the coil13 it would flow in the opposite direction. This is explained by thefact that the two coils form a closed circuit at all times, but duringclosure of the key T energy is stored in the coil of relay A having thehigher reactance and is released upon opening the key T so as todischarge through the less reactive coil and in a direction opposite tothat of the normal current. Obviously the absolute current variation isthe smallest in the branch which has the highest self induction. On theother hand one and the same absolute current variation would even leadprimarily to a reversal of the current in the branch in which at firstthe minor current flows, i. e., in the branch having the highest ohmicresistance. Both conditions have the result that, after disconnectingthe voltage source, the current in the coil having the higher selfinduction and lower ohmic resistance, hence the current in the coil A,would decrease in fact but it will retain its direction, while thecurrent in the other coil must reverse its direction.

the coil in which the ratio between the self induction and the ohmicresistance is the smaller.

Since therefore on opening of the key the current passing through thecoil B reverses its direction, the contact I) would open at the most fora very brief period and the closure periods would be unduly prolonged.Hence a rapid switching of the relays and unbiased keying is notpossible. But the introduced rectifier G has the effect that thede-energization current meets such a high resistance that the relay Bwill be unaffected thereby, and, furthermore, the relay A willde-energize more rapidly. Therefore, im-

proved keying performance is obtained.

In order that the rectifier may not be subjected to an excessive load inthe direction of blocking, it is advisable to insert additionally inparallel with the relay coil A an ohmic resistance R A closer reasoningshcws that the current can be reversed only inthrough which the highpotential appearing at the coil A on opening the switch T will besubstantially diminished.

The invention is applicable wherever two different inductive resistancesare to be connected in parallel and a rapid decrease of the current isdesired on disconnecting the direct voltage. If more than two inductiveresistances are employed, such rectifiers should be inserted in allparallel branches with the exception of the branch which contains theresistance having the highest ratio between self induction and ohmicresistance since in this branch the current retains its existingdirection while in the other branches the direction of the current canreverse itself.

Iclaim:

1. A keying circuit having a pair of parallel connected relays, thewinding of one relay being more highly reactive than the other, andmeans 20 including a rectifier in series with the relay winding havingthe lower reactance for preventing current reversals in its winding dueto the gradual dissipation of energy stored in the winding of the morehighly reactive relay.

2. A keying circuit having a code signal relay of relatively lowreactance, a relay of relatively high reactance connected in parallelwith said code signal relay, a utilization circuit under control of saidhigh reactance relay, a key and a direct current source common to saidrelays for energizing the same, means including a unidirectionalconductor in series with only the winding of said code signal relay forpreventing the dissipation through its winding of energy stored in therelay of relatively high reactance, and a resistor in shunt with thelast mentioned relay for dissipating the energy which remains storedtherein after opening said key.

FRITZ HAHNDORF.

